Enhancing Detection Sensitivity of Responsive Microgel-Based Cu(II) Chemosensors via Thermo-Induced Volume Phase Transitions

We report on the fabrication of responsive microgel-based Cu 2+ chemosensors possessing tunable detection sensitivity via thermo-induced microgel collapse/swelling. A novel phenanthroline-containing fluorescent monomer capable of Cu 2+ -binding and fluorescence sensing, PhenUMA ( 4 ), was synthesized at first by reacting 5-amino-1,10-phenanthroline ( 2 ) with 2-(3-isocyanato-propionyloxy)ethyl methacrylate ( 3 ). Near-monodisperse Cu 2+ -sensing microgels were synthesized via emulsion polymerization of N -isopropylacrylamide (NIPAM) in the presence of N, N ′-methylenebis(acrylamide) (BIS), an anionic surfactant, and PhenUMA ( 4 ) at around neutral pH and 70 °C. At 20 °C, PhenUMA-labeled microgels in their swollen state can selectively bind Cu 2+ over other metal ions (Al 3+, Mg 2+, Zn 2+, Fe 3+, Mn 2+, Ni 2+, Ag +, Cd 2+, Hg 2+, and Pb 2+ ), leading to prominent quenching of fluorescence emission intensity. At a microgel concentration of 0.25 g/L, the Cu 2+ detection limit can be down to ∼125 nM. When heated above 32 °C, fluorescence intensity of PhenUMA-labeled microgels in the absence of Cu 2+ exhibits an approximately 33% increase due to their volume phase transition, which is reasonable considering that fluorescent PhenUMA moieties are now located in a nonpolar environment. Furthermore, Cu 2+ detection sensitivity of PhenUMA-labeled microgels can be dramatically enhanced via thermo-induced microgel collapse at elevated temperatures. At a microgel concentration of 0.083 g/L, detection limits of Cu 2+ ions can be drastically improved from ∼28 nM at 20 °C to ∼8 nM at 40 °C. A plausible mechanism for the thermo-induced enhancement of Cu 2+ detection sensitivity has been proposed. To the best of our knowledge, this proof-of-concept work represents the first example of responsive microgel-based metal ion chemosensor with thermo-tunable detection sensitivity, which simultaneously combining advantageous properties of small molecule sensing moieties and stimuli-responsive soft matter entities.